In an electrophotographic engine, a primary imaging member (PIM) such as a photoreceptor is initially charged using known means such as a grid controlled corona charger or roller charger. An electrostatic latent image is then formed on the PIM by image-wise exposing the PIM using known means such as a laser scanner, an LED array, or an optical exposure. The electrostatic latent image is converted into a visible image, also referred to as a toner image by bringing the latent image bearing PIM into close proximity to a development station containing dry toner particles, also referred to as marking particles. The toner particles are electrostatically charged and the bias on the development station, relative to that in the image areas of the PIM is set so that a desired amount of toner is transferred from the development station to the PIM.
The toner image is then transferred from the PIM to a receiver such as paper by pressing the receiver into contact with the image-bearing PIM while exerting an electrostatic field so that the toner particles are urged to the receiver. The toner image can be transferred directly to the final receiver directly. Alternatively, the toner can be first transferred to a transfer intermediate member and then transferred from the transfer intermediate member to the final receiver. The toner image is then permanently fixed to the receiver by fusing the image, generally accomplished by subjecting the image-bearing receiver to a combination of heat and pressure sufficient to raise the toner to a temperature in excess of its glass transition temperature Tg and allowing the toner particles to flow into a cohesive mass. The PIM is cleaned after transfer to remove residual toner and other contaminants and made ready to produce another print.
To produce a color print electrostatic latent images are produced on a photoreceptor and then converted into color separation images corresponding to the subtractive primary colors, generally cyan, magenta, yellow, and black. These color separation images are transferred in register to a final receiver such as a sheet of paper. Transfer of the toner images can be done by either transferring to an intermediate transfer member and then from the intermediate transfer member to the final receiver or to the final receiver directly from the PIM. If a transfer intermediate member is employed, the separations can be transferred either in register to the intermediate transfer member or to separate transfer members and then transferred in register to the final receiver. Alternatively, the toner separations can be transferred to a single intermediate transfer member or to separate portions of the intermediate transfer member and then transferred, in register, to the final receiver.
In order to convert an electrostatic latent image into a visible image and then transfer the toner used to convert the electrostatic latent image into a visible image to a receiver, the toner particles must possess a carefully controlled electrostatic charge. This is accomplished by mixing toner particles with magnetic carrier particles to form a developer. The toner particles tribocharge against the carrier. To enhance and control tribocharging, the toner and carrier particles may comprise charge agents such as those known in the literature. The types and concentrations of the charge agents, in addition to the electronegativity properties of the toner and carrier, will result in a controlled, uniform charge being imparted on the toner. In addition, charge control can be further enhanced using particulate addenda on the surface of the toner particles.
The charge of the toner, expressed as the toner charge-to-mass ratio, can be determined using a method such as that described by J. C. Maher, Proc. IS&T's Tenth International Congress on Non-Impact Printing, IS&T, Springfield, Va. (1994), pp. 156-159. The apparatus consists of two parallel disk electrodes with a separation of 1.0 cm. The top electrode is connected to an electrometer. The bottom electrode is connected to a voltage source. A rotating segmented circular magnet is underneath the bottom electrode. Developer is placed on the bottom ring and a potential is applied between the electrodes as the segmented magnet is rotated. Motion of the developer due to the rotating magnet detaches toner from the magnetic carrier. The free toner is deposited on the upper electrode and the integrated charge associated with the deposited toner is measured by the electrometer. After a sufficient time (about 30s) the upper disk is removed and passed under a magnet to remove stray carrier. The weight of toner on the disk is determined to obtain the charge-to-mass ratio.
Carrier particles typically comprise a magnetic material such as iron, ferrite, etc. The carrier particles can be either a soft or hard ferrite.
The size of the particulate addenda appended to the toner particles can be determined, for example, using the nitrogen absorption method of Brunauer, Emmett, and Teller (J. Am. Chem. Soc. 60, 309 (1938), commonly referred to as BET. A suitable instrument for determining the size of the particulate addenda is the Quantachrome Monosorb manufactured by Quantachrome Corporation.
Terms such as “toner diameter” and “carrier diameter” can refer to the median volume weighted diameters of the toner and carrier, as determined using a commercially available instrument such as a Coulter Multisizer. In years past, toner particles had diameters greater than 12 μm and often greater than 20 μm. However, for reasons that will be described presently it has proved difficult to generate to generate high resolution toner images using such large toner particles, accordingly, modern toner particles have diameters of approximately 6 μm to 8 μm.
In particular, it will be understood that larger toner particles are difficult to transfer causing toner images made with larger particles have poor resolution and high granularity. One reason for this is that the Coulombic repulsion between large toner particles causes such larger toner particles to fly apart during transfer thus degrading image quality. This effect is known in the art as dot explosion. In addition, it will be appreciated that the amount of charge that can be formed on the PIM is limited according to material properties of the PIM and the amount of large particle toner that can be transferred to the PIM during development is therefore limited due to higher charge levels required to transfer such large diameter toner particles.
In contrast, small toner particles can be more controllably deposited onto the PIM and have higher resolution and lower granularity. In addition, the Coulombic repulsion tends to cause less scatter of the toner particles, reducing dot explosion. However, small diameter toner particles are more difficult to electrostatically transfer and, in fact, generally require the addition of small particulate addenda such as silica to enhance transfer.
Typically, developer comprises toner and carrier particles in a ratio of between approximately 2% and 12% by weight, depending on the size of the toner and carrier particles. The developer is loaded into a development station that contains and electrically bias able magnetic brush. The magnetic brush contains a core of magnets, generally alternating in polarity and a shell onto which the developer is brought into close proximity with the PIM so as to allow toner to come into contact with the PIM and convert the electrostatic latent image into a visible image. To bring fresh developer into the nip formed between the shell and the PIM, either the shell, the magnetic core, or both rotate. This rotation subjects the toner particles to centripetal accelerations such that, if the toner charge to mass ratio is too low, the toner will be thrown from the carrier and result in the formation of an undesirable powder cloud in a process known as dusting. The amount of toner deposited on the PIM depends on the difference of potential between the development station and the appropriate portion of the PIM, as well as the toner charge, with higher charged toner being deposited less than lower charged toner. However, if the charge on the toner particles is too low a condition known as dusting will result in which all portions of the PIM are being coated with toner. This would result in undesirable image background.
The term “mass of a toner particle” or mt refers to the mass of a spherical particle of the same material and having a radius equivalent to half of the toner diameter. Toner typically comprises a polymeric binder such as polyester (mass density ρ=1.2 g/cm3) polystyrene (mass density p=1.0 g/cm3), etc. The mass of a toner particle is calculated assuming a spherical particle of equivalent diameter. The mass of a toner particle is thenm=4/3πR^3ρwhere R is the radius of the toner particle and p is the mass density of the polymer binder. The charge on a toner particle q is the charge-to-mass ratio of the toner times the mass of a toner particle. It is apparent that centripetal acceleration varies as the cube of the toner particle radius.
As discussed, toner charge in a two component developer is generated by tribocharging the toner particles against the carrier. Accordingly, the charge on the toner depends on the surface area of the toner particle that is capable of contacting the carrier. While surface area can be accurately measured using BET, the amount of available surface area can be approximated using the surface of a spherical particle of equivalent radius, orA=4πR^2.
The charge to mass of the toner would, accordingly, vary approximately as 1/R. Thus, large toner particles would have a higher charge than would smaller ones, but the charge to mass ratio of the larger toner particles would be smaller for a constant set of materials.
Another force that needs to be considered in transferring toner and maintaining dot stability are the van der Waals forces. These van der Waals forces give rise to the adhesion forces between the toner particles and any contacting substrate such as the PIM. They also give rise to cohesion between toner particles that stabilizes toner structures such as alphanumerics and half tone dots against disruption caused by Coulombic repulsion between particles. These forces are known to increase linearly with the toner radius, as discussed by Rimai et al. J. Imaging Sci. Technol. 47, 1 (2003).
It is often desired to produce a dry electrophotographic image with both small and large toner particles. For example, such a combination can be used to create image texture or relief, wherein the small toner particles are colored and serve as marking particles and the larger toner particles are clear and serve to allow texture to form. However, this is especially problematical. The presence of large toner particles can disrupt the formation of a toner image on the PIM due to its high charge and mass. In addition, with large toner particles, image disruption tends to be quite pronounced due to the Coulombic repulsion dominating over the van der Waals attraction. This can aggravate dot explosion. Moreover, the presence of large toner particles can impede the transfer of the small toner particles. Specifically, transfer is accomplished by applying an electrostatic transfer field E to urge the particles towards the receiver. However, the maximum applied field that can exist across an air gap, known as the Paschen discharge limit of air, varies inversely with the size of any air gap. Within a transfer nip formed by donor and receiver members, the gap is determined by image characteristics such as the toner diameter whereby the toner particles serve as tent poles that separate the two members.
Finally, transfer of small toner particles, i.e. toner particles having diameters less than 12 μm and generally between 2 μm and 8 μm is limited because the van der Waals forces are greater than the applied electrostatic forces. While the applied electrostatic force might be increased by increasing the toner charge, this would adversely affect the amount of toner that can be deposited in development. Moreover, the electrostatic image force between the toner and the primary imaging member increases as (q/R)2, making transfer more difficult. Finally, in transferring a color image, high charge on a previously transferred toner image would decrease the applied transfer field available to transfer a subsequent image.
As shown by Rimai et al. J. Imaging Sci. Technol. 47, 1 (2003), van der Waals forces can be decreased by appending small particulates to the surface of the toner and the use of such addenda is required to transfer small toner particles. However, as shown by Rushing et al. (J. Imaging Sci. Technol. 45,187 (2001)) and by Gady et al. (J. Imaging Sci. Technol. 43, 288 (1999) increasing particulate addenda increases dot explosion and decreases resolution. However, the use of such addenda is required to transfer small toner particles. For larger toner particles, those with diameters greater than 14 μm and even more so for toner particles having diameters greater than 20 μm the use of particulate addenda is generally not desired as the applied electrostatic forces dominate over van der Waals forces and the application of such addenda would decrease toner cohesion, thereby aggravating dot explosion.
At a minimum, challenges associated with transferring large toner particles limits the amount of large particle toner that can be transferred during a single pass and also causes a lack of coherency in the large particle toner that is transferred. These effects, in turn, limit the height of a toner stack that can be formed using large toner in a single toner transfer operation. However, it is desirable to be able to create toner stack heights in a single pass that are as high as is possible as this enables the creation of inverse mask toner patterns, structures having a distinct tactile feel, and other structural or aesthetic features that can be formed using relief patterns on the surface of a receiver without requiring multiple passes through the printer. Further, it is desirable to allow the creation of toner stack heights having improved packing densities of toner to provide more homogenous and more resilient toner structures.
It is clear that, to form an image that combines of small and large toner particles, new processes and materials are needed.